1. Field of the Invention
This invention relates to electrostatic powder painting, and more particularly to the configuration of spray guns used for hand spraying.
2. Description of the Prior Art
In electrostatic powder painting, dry paint particles are fluidized in a powder hopper and pumped through a hose to a spray gun which sprays the powder onto anitem or part to be coated. The spray gun typically charges the powder in one of two ways--either the gun has a high voltage charging electrode, or the gun has means to charge the powder by friction, i.e., tribo-electrically. This invention relates to both types of spray guns but will be described with respect to tribo-electric type powder spray guns.
Generally, in tribo-electric powder guns, the powder is epoxy based, and surfaces are provided within the gun, typically constructed from polytetrafluoroethylene (PTFE), which the powder particles impact numerous times to frictionally charge the particles. When the powder particles are sprayed from the front of the gun, they are electrostatically attracted to the product to be painted which is generally electrically grounded. Once these electrostatically charged powder particles are deposited onto the part, they adhere there by electrostatic attraction until they are conveyed to an oven where they are melted to flow together to form a continuous coating on the part. Powder coating generally provides a tough and durable finish such as would be found on many appliances, garden furniture, lawn mowers, and other products.
Electrostatic powder spraying can be performed either automatically or manually. Automated powder spraying is often preformed on production lines in which the part to be sprayed is conveyed through a spray booth, and automatically controlled spray guns, which are mounted at fixed positions, on robots or gun movers, at appropriate locations in the spray booth, spray the powder onto the parts in accordance with a preset automated program. Manual powder spraying is usually performed by one or more operators who hold the spray gun in their hands and spray the powder on to the parts by hand. Effective manual powder spraying relies upon the skill of the operators and their ability to visually observe where the powder should be sprayed and their ability to operate the spray gun to deposit the powder in the desired locations on the parts.
U.S. Pat. No. 5,344,082 shows an improved tribo-electric powder spray gun which is used in automated spraying operations. This gun is available as a Tribomatic.TM. II gun from Nordson Corporation. In this gun, an improved powder flowpath is provided using the arrangement of a core within a sleeve, wherein the powder flowpath is provided between the exterior of the core and the interior of the sleeve. The exterior of the core and the interior of the sleeve are provided with undulating or wavy PTFE surfaces, so that an annular wavy flowpath for the powder is provided within the gun. This causes the powder to change direction and contact the PTFE charging surfaces numerous times while passing through the charging portion of the gun, with the powder particles picking up charge on each contact. The exterior of the core and interior of the sleeve are held to a close tolerance so that the powder flowpath is very narrow, further increasing the number of times each powder particle hits a charging surface. The core can be easily inserted into and removed from the sleeve for assembly and repair, and this removability is accomplished by dimensioning the diameter of the peaks or ridges of the inner core to be less than or at most equal to the diameter of the peaks or ridges of the outer sleeve. In addition, the wear sleeves on both the inner core and the outer sleeve are longitudinally symmetrical, so that the gun can be re-assembled with either end of the wear sleeves inserted first. A diffuser is provided at the rear of the gun to control the charge on the powder by driving the powder through the charging portion at the desired velocity.
The automated spray gun in U.S. Pat. No. 5,344,082 also provides a simplified grounding path that avoids the time consuming and complicated manufacturing process by incorporating a ground ring at the beginning of, but outside of, the powder flowpath. By placing the ground ring outside of the flowpath, the ground ring is kept clean. In addition, by placing the ground ring at the inlet to the charging portion of gun, the ground ring is located where the greatest rate of charging occurs, and this location is the ideal place to bleed off charge.
These features are disclosed in U.S. Pat. No. 5,344,082 in connection with an automatic spray gun. There is a need for a spray gun having these features that can be hand held and used for manual spraying as well as a need for improved manual spray guns generally.
Hand-held powder spray guns have traditionally been configured in a design resembling a pistol with a handle extending downwardly from the body of the spray gun. The operator grips the handle as he would a pistol and points the charging portion or barrel of the spray gun at the part to be sprayed. While the pistol handle configuration has long been used for powder spray guns, it may not be ergonomically appropriate when the spray gun is used in certain positions by the operator. For example, when the operator is spraying a portion of the part down low in the spray booth, he must hold the pistol handle spray gun even lower, and this awkward position may make it exceeding difficult to spray effectively. Depending upon the skills of the operator, the spraying of the lower portion of the part may lack proper quality due to the inability of the operator to perform proper spraying when using the gun in such awkward positions.
In addition, there is a need for automatic spray guns which can be quickly and easily converted to manual spray guns having ergonomically designed handles to facilitate operator comfort.